From the PL website:
They check everything that is potentially a red card. The way you say would mean they aren't allowed to check stuff the ref hasn't seen which wouldn't make any sense and going against a large part of VAR is for. They check anything that could possibly be a red card, whether the ref gave a red, yellow, or missed it entirely. They check it.
They have told you, it is on their website that they check every possible red card situation. What more do you want? Refs don't publicly tell everyone every thought and decision they have and make during a match, why would you expect VAR to be any different? The rule is to check it so they checked it. They are actively watching the match, not staring at a wall waiting for the ref to ask them to watch a replay. You've either misunderstood how it works or been told the wrong information.